Accelerometers are used to measure the acceleration of piles being driven into the earth by a pile driving hammer. This information is useful in measuring the efficiency of the pile driving hammer, as well as driving resistance of the pile, along with other useful measurements. Primarily, driving resistance is significant in that it must reach a predetermined level in order for the pile to adequately bear the desired load.
Conventional systems for measuring driving resistance and pile velocity utilize self-generating-type accelerometers, which, as the name implies, self-generate direct current electric signals. A quartz or piezoelectric crystal is compressed by the mass of the accelerometer during movement of the pile, producing electrical impulses proportional to the acceleration of the pile. The acceleration signals produced by the accelerometer are recorded and subsequently electronically integrated on separate equipment to produce a velocity measurement. The velocity measurements are in turn, electronically integrated a second time to produce a measurement of pile displacement. The number of recorded blows are determined for each linear unit of displacement to arrive at a "blow count". The "blow count" determines an average displacement per hammer blow for the pile. The force applied to the pile by the hammer is sensed simultaneously by separate apparatus. The force and average displacement can be related to driving resistance by known formulas which recognize soil conditions, pile configuration and desired depth of penetration. This final calculation is currently performed manually, usually on a "bearing graph".
However, conventional systems suffer from general inherent limitations. Specifically, self-generating accelerometers sometimes display a "zero offset error" in the acceleration signal during or just after the hammer blow to the pile. FIG. 1A graphically represents, in Line A, the force applied to a pile during a hammer blow. FIG. 1B graphically illustrates, in Line B, an accurate representation of the velocity of the pile as a result of the hammer blow. Line C in FIG. 1B shows a ramped portion constituting the integral of the "zero offset error". This ramp error distorts the observed displacement of the pile when the velocity signal is integrated, prevents an accurate measurement of driving resistance, as well as increasing or decreasing the apparent number of hammer blows depending on the direction of the ramp.
As a result, it has been customary to visually observe displacement of the pile and to manually record blows per unit of displacement to arrive at a "blow count". This method is inherently subject to human error and reduces the reliability of the computation of driving resistance, and is therefore unsatisfactory. Further, it is not possible using this method to calculate the driving resistance for a selected hammer blow, only an average figure over a certain amount of displacement.
Self-generating accelerometers, because they operate by generating direct current electric power, also inherently generate a displacement error in the acceleration reading caused by rapid movement through the ambient magnetic field.
Therefore, it is an object of this invention to provide improved method and apparatus for measuring the driving resistance and velocity of piles during driving.
It is another object of this invention to provide such an improved accelerometer that does not exhibit a zero offset error and displacement error when measuring the acceleration of piles being driven, thereby allowing accurate measurements of the blow count, velocity and displacement imparted to the pile by the hammer blow.
It is yet another object of this invention to provide an improved method and apparatus for measuring the driving resistance and velocity of a pile for selected individual hammer blows during driving.
Therefore, these and other objects and advantages of this invention will be apparent to those skilled in the art from a consideration of this specification, including the attached drawings and appended claims.